Shikwamkwa Replacement Dam

The original Shikwamkwa Dam near Wawa in northern Ontario was part of the Hollingsworth Generating project. It was built in 1958 as a zoned, earth-fill structure on a complex, highly pervious foundation.

At the time of construction, the subsurface geological conditions were not fully understood and the measures for seepage control proved to be inadequate. As a result, serious foundation problems developed immediately following impoundment and they progressively worsened. As the foundation conditions deteriorated, seepage flows increased, deep sinkholes formed in the head pond, active boils formed downstream and discharges of fine grained materials periodically occurred. These were all signs of a serious and developing foundation problem.

In 1994, Brookfield Power asked Hatch Energy to recommend a course of action to mitigate the risks posed by this ageing structure. Key to the success of this approach was the installation of a fully automated real-time instrumentation monitoring system designed to collect subterranean temperature data, piezometric pressures, and weir flow data. The system was very advanced at that time.

A critical issue was the release of fine-grained materials downstream of the dam, suggesting that “piping,” a type of internal erosion, had been occurring sporadically for many years. To track these random events, an innovative, fully automated, turbidity monitoring instrument, the first ever developed for use in dam safety assessments, was designed. It was able to meet the site conditions at the Shikwamkwa Dam and withstand the rigours of the harsh Northern Ontario climate.

From 1994 to 2003, continuing analysis showed the dam was behaving in a more or less predictable fashion, allowing a phased approach to repairs to continue. However, in 2003, a subtle, but important, change in the results indicated that the foundation was in flux. Hatch Energy responded by developing a 3-D groundwater seepage model, calibrated to account for the complex geological conditions and the alteration of the foundation that had occurred.

Hatch Energy also developed a sonar profiling tool that was capable of locating and accurately mapping a small diameter sinkhole, only 1 metre in diameter, but one that extended over 30 metres directly beneath the dam.

Decision to replace

It was concluded that the only appropriate long-term solution was the construction of a new structure. This new dam was to be 300 metres downstream of the original dam, at a location where it was possible to seal the foundation adequately. The project schedule was ambitious: the problem had to be rectified before the onset of the 2006 spring freshet.

Constructing the new embankment dam involved placing and compacting over 1,000,000 cubic metres of earth fill. At 38 metres high and approximately 1 kilometre long, it is the largest dam constructed in Ontario in decades.

Below the structure, the difficult and complex foundation was sealed using a plastic concrete “cutoff wall.” The wall extends to depths of 65 metres, making it one of the deepest cut-off walls in the world. It was constructed by excavating a narrow 0.75-m diameter trench through the pervious overburden foundation down to the bedrock surface. The trench was supported during construction by a thick bentonite clay slurry. The slurry was subsequently displaced by placing plastic concrete from the bottom up. The dam was then built over this wall.

Computational, grouting and mixing innovations

Advanced computational techniques that incorporated detailed geotechnical information as it became available enabled the designers quickly to turn around modifications to the design. The original dam was still being monitored, as it was serving to hold back 30 metres of water while the replacement dam was constructed.

During construction, a contact grouting program was devised to seal the base of the cut-off wall to the irregular bedrock surface. Quality assurance involved the first use of a routine, on-site, triaxial compression testing program to assess both compressive strength and the flexibility of the wall at simulated critical confining pressures.

Hatch Energy also devised an innovative mixing technique to produce an enriched till for use at the critical interface of the top of the cutoff wall and the impervious core of the dam, a technique that saved costs and proved very effective.

During final closure of the cut-off wall, analyses with the seepage model allowed the pressure relief systems to be reduced from the original design, saving considerable costs.

Construction was completed five months ahead of schedule, millions of dollars below budget and with no lost time injuries.

Hydro-Qubec visited the project to learn from the groundbreaking work that was being performed at this remote northern Ontario site. The post construction performance of the dam is in line with the 3-D seepage analyses, and the seepage flows were reduced from 600 to 1,000 L/sec to 30 to 100 L/sec.CCE

Name of project: Shikwamkwa

Replacement Dam, Wawa, Ont.

Award-winning firm/prime consultant: Hatch Energy, Niagara Falls, Ont. (Geoff Rigby, P.Eng., Steve Rigbey, P.Eng., C. Richard Donnelly, P.Eng., Darren Protulipac, P.Eng.,

Warren Hoyle, P.Eng.)

Owner: Brookfield Power Corporation

Environmental consultant: Natural Resources Solutions (NRS)

General Contractor: Peter Kiewit Sons